This invention relates to stereoscopic motion pictures and specifically to an improved means for the presentation of single-strip dual image stacked frame stereoscopic films projected with standard theater equipment operated by regular theater personnel.
Heretofore there have been proposals to create stereoscopic motion pictures by incorporating into the area of one normal film frame two separate images comprising similar but not exactly duplicate views of the same scene as would be seen by the right and left eyes of an observer. These twin images are known as a stereo pair. The size, placement, and/or location of the twin images within the confines of the film frame can vary. The most popular being one image placed above the other in a "stack", usually with the left eye image stacked on top of the right eye image, which yields a pleasing wide picture shape. Another popular configuration is to place the twin images one beside the other, while less popular are side-by-side and rotated on end or foot-to-foot, as well as a stacked configuration in which one image is upside-down and/or reversed left-to-right (mirrored) in relation to the other. Providing motion picture films with such configurations are basically simple matters, since relatively few cameras, specialized lenses, or optical printer systems are required during the production of such a film whereas thousands of theaters will ultimately have to show them. Camera equipment is often drastically altered without regard to returning it to normal operation later. Exhibitors, on the other hand, are exceedingly reluctant to alter or sacrifice their projection equipment in any way since these are "bread and butter" machines which must be used day after day on a continuing basis.
There are a number of stereoscopic filming and/or printing processes that yield a stacked-frame configuration. No two brands are exactly the same, and the resultant film frame format specifications thus produced vary considerably.
The presentation of these specialized dual image films to an audience likewise requires the use of special projection equipment.
The criteria for projecting the images, in a manner that the stereoscopic effect can be perceived by an audience, are well known. The twin images must be brought into a superimposed relationship and correctly registered on the viewing screen for accommodation by the eyes. The separate images on the screen must be rendered visible only to the one eye it is intended for, and the other "wrong" image must be concealed from view by that eye. This is usually accomplished by the action of light filtering mediums or materials such as polarizing filters which are used extensively. Two polarizing filters with opposing axis are each placed in one of the image forming light beams. Viewing spectacles with each lens containing a polarizer duplicating the axis arrangement of one of the projection filters are worn by the audience members. The action of the polarizers is such that one image only is visible through one of the spectacle lenses, while the un-wanted image is blocked from view to that eye, and vice versa for the other eye. Thus the right and left eyes can see only the correct image intended for them to see. Unfortunately, the technicans responsible for the camera taking equipment or special printing procedures exhibit little concern in regard to the ultimate presentation of the film, and the means for projection are often an after thought. Often these skilled engineers consider the state of the art and sometimes specialized models of projection equipment found in their laboratories to be typical of the theatrical profession. Thus the equipment which evolves for the projection of any given stereoscopic process is seldom practical for the working projection room environment. The situation involving the presentation of the stereoscopic film to the audience worsens when the absence of knowledge by the regular theater staff, who are not accustomed to dealing with such optical phenomena, is taken into account. Thus, the theater staff are embarrassed by an unprofessional performance and the audience is exposed to considerable viewing distress.
It has been proposed to project the dual image film with special double lens system arrangements housed in a single lens barrel which replaces the standard projection lens completely. The optical centers of the dual projection lens element segments must correspond precisely with the optical axis centers of the twin images on the film. Such a lens system must either be made adjustable to suit the various image center-to-center specifications on the films or an army of separate pre-formulated lens systems must be provided for use with all the slightly differing dual image formats each of which requires its own awkward installation procedure. The polarizing filters must be fitted in close proximity to the outermost lens element in the hottest part of the optical train. Often the oddly shaped dual lens system does not properly fit into the projector's lens mount or receptacle, requiring hasty alterations to the machine. The lens element segments used in the construction of such a system must, out of necessity, be smaller than those found in a full size normal lens, and the unusual semi-circular shape results in an overall low "f" ratio causing considerable light loss on the theater screen. This light loss is further compounded by the production of complete but unwanted repeat or secondary images appearing on the theater screen and auditorium front wall above and below the desired stereo screen image. These must be physically masked from view and the light devoted to their production considered as a general but large loss to the entire screen image. The polarizing filters absorb another large portion of light. Being placed close to the lens elements due to optical requirements also places them in the hottest possible location. Polarizing filters are prone to fading due to their heat sensitive nature. The loss of the filtering action caused by the "burn out" results in the screen images reverting to the naked eye appearance, blurred and garbled, the spectacles worn by the audience having, in effect, become useless. Replacing the ruined filters can be quite a job and entails another lengthy installation procedure. Heat filters inserted into the light beam prior to the polarizers are of limited value, absorb even more light, and yield a definite blue-green coloration of the screen.
Other proposals involve the use of wedge prisms used in conjunction with a standard projection lens. Two wedge prisms are placed together base-to-base forming a bi-prism and inserted into the light path of the screen image. Their refractive qualities tend to divert the top portion of the image downwards and the bottom portion of the image upwards as the image forming light beam progresses to the theater screen. Polarizing filters are inserted in the image path between the wedges and the theater screen, with axis oriented to match the spectacles worn by the audience for appreciation of the stereoscopic effect. The refractive power of the wedges determines the focal length of the standard projection lens, size of the theater screen, and distance from the booth which can be used. A small degree of adjustment can be obtained by pivoting the wedges in unison. It is only by careful calculation that the requirements of a particular theater's system size can be fulfilled for a proper stereoscopic display. The wedge prisms must be achromatic or the screen images will be rendered as a spectrumated smear or blur. Wedge prisms, achromatic, apochromatic, or plain glass all introduce an irritating compound optical distortion in the form of curving or bowing lines plus a non-uniform compressing effect, and due to the base-to-base disposition of the wedges this bowing distortion is reversed in respect to each of the dual images. Because of this distortion it is impossible to attain a suitable superimposed registration throughout the entire screen image. This bowing distortion is accentuated as the wedge prisms are pivoted for adjustment of the dual images into proper register on the screen. The wedge prisms themselves are expensive and difficult to make. Each achromatic pair requiring at least 4 large bubble and strain free glass blanks, 8 surfaces to be optically figured, ground, and polished to a very high degree, and a critical-glass-to-glass cementing procedure. They must be of sufficient size to permit easy access of the entire screen image light beam to pass through them or a considerable light loss due to vignetting results. Such projection devices are large, heavy, awkward, and can cause damage to the lens holder. They are best mounted independently of the projection machine.
Other proposals involve the use of mirrors or reflecting surfaces used in conjunction with a standard projection lens. The purpose of which is to cut the screen image light beam into two halves, each half containing one of the dual images, and re-positioning the halves together in proper superimposed registration on the screen. The present invention is in this category. Most such proposals are usually associated with non-professional uses, intended primarily for amateur stereo slides and home movie applications. In the few proposals directed toward commercial stereoscopic applications the devices were hastily designed and poorly made. and hastily designed. The optical principles and qualities were not utilized to the fullest extent as found in similar high grade mirror optical components used in science and industry elsewhere.
Accordingly the several objects of my invention are to provide a means to easily, properly, and professionally project or present to a commercial theater audience any and all of the differing single film strip stacked-framed format type stereoscopic film productions, regardless of the varying specifications or center-to-center spacing of the dual image pairs.
Another object is to provide this means of stereoscopic presentation without substituting projection lenses, altering or requiring mechanical changes to the standard theater projector which the theater personnel are accustomed to operating.
Still another object is to provide this means of stereoscopic presentation without the necessity of removing and replacing the stereoscopic projection device for the presentation of non-stereoscopic program material.
Another object is to provide the means to easily project most of the single strip side-by-side format type of stereoscopic program material.
Another object is to provide the means to substantially prevent polarizing filter failure due to over heating.
Another object is to provide the means to quickly rectify the effects of a polarizing filter failure if it should happen, or the filter fall victim to other forms of damage, without interruption to the program.
Another object is to provide substantial construction with direct and easy-to-use operating controls governing all the members of the projection device to remedy unusual circumstances that may arise.
Another object is to provide an attaching a means to positively lock and unlock the projection device to and from the projection machine in a quick, easy and repeatable fashion.
Further objects and advantages of my invention will become apparent from a consideration of the drawings and ensuing description thereof.